Add aconfig flag for fixing the missing HUN removal animation for group

Add aconfig flag for fixing the missing HUN removal animation for HUNs
that are posted as a group, but not showing as a group HUN.

Bug: 343475993
Test: n/a
Flag: com.android.systemui.notification_group_hun_removal_animation_fix
Change-Id: Ic997737c839974a6710739d868910750f4d9ff39